It is common to design agricultural vehicles, such as tractors, with a so-called structural engine. In such vehicles, the engine, together with the transmission and the rear axle, constitutes the rigid chassis of the vehicle. Because there is no suspension between the chassis and the ground, at least at the rear of the vehicle, shocks resulting from unevenness in the terrain are transmitted directly to the driver's cab.
To improve the comfort of the driver, it is known to pivot the cab about a transverse axis at its front end and to provide spring and damper units between the rear end of the cab and the chassis. This allows a cushioned up and down movement of the cab on the chassis.
While such a cab suspension goes some way towards improving the ride quality in the cab, it still suffers from the disadvantage that when the tractor is being driven along an incline, the cab and the driver's seat, though parallel to the ground, are inclined relative to the horizontal and this causes discomfort to the driver, aside from being disconcerting.
It is therefore desirable to provide the cab of an agricultural vehicle with a self-leveling support system which maintains the attitude of the cab substantially horizontal, even if the inclination of the ground over which the vehicle is travelling varies within certain limits.
In U.S. Pat. No. 6,273,203 B, there is disclosed a self-leveling support system that employs four hydraulic actuators capable of tilting the cab about mutually inclined axes and a control system for independently controlling each of the hydraulic actuators in dependence upon signals received from sensors, which can be constructed as inclinometers or gyroscopes.
In this known system, the fact that all four actuators can be independently controlled makes for a complicated control system. This is because the control algorithm needs to take into account of when an actuator is near the bottom or top of the stroke of an actuator and is therefore unable to bring about the required tilt of the cab. Furthermore, a powerful pump is required to be able to supply hydraulic fluid to four actuators sufficiently rapidly to counteract rapid changes in the inclination of the chassis.